A technique is known that when a chromium nitride film is formed on a metal surface, the wear resistance and the corrosion resistance of the metal is improved. Thus, for the purpose of lifetime enhancement of a mechanical member, a mold tool, a machining tool, or the like, this chromium nitride film formation is widely employed.
In general, the chromium nitride film is formed by a physical vapor deposition (PVD) method such as: an ion plating method that in a vacuum chamber, Cr is vaporized, ionized, and then projected onto a substrate and, at the same time, nitrogen gas is introduced so that the chromium nitride film is formed; and a sputtering method that a high voltage is applied between a target and a substrate so that glow discharge is generated and Ar ions of the obtained plasma are caused to collide with the target surface and thereby Cr atoms are sputtered and deposited onto the substrate.
However, when such a chromium nitride film formed by the PVD method is employed as a surface treatment layer of a high-load slide member such as a chain, the chromium nitride film easily spalls from the metal substrate of steel or the like and hence there has been a problem of difficulty in causing the chromium nitride film to adhere to the surface of the substrate and thereby to unite with the substrate such that spalling hardly occurs.
Then, in the PVD method, in some cases, droplets occur on the surface. The droplets cause a problem that the surface roughness increases and cracks occur with starting from the droplets so that the wear resistance is degraded. Even when the droplets are removed by polishing, holes are formed in the portions where the droplets are removed. Then, the holes grow larger in association with the load and then the holes are connected together so that a problem is caused that the wear resistance is not allowed to be improved.
Further, the PVD method has a problem that lifetime enhancement by increasing the film thickness is not allowed to be performed because this approach causes occurrence of cracks and hence degradation in the wear resistance.
Further, there has been a problem that if the to-be-processed material has a hole, film formation up to the inner face of the hole is difficult.
Examples of the above-described mechanical member include a pin provided in a timing chain for automobile engine. Examples of such a timing chain include a roller chain, a bushed chain, and a silent chain.
A roller chain is constructed such that both end parts of a cylinder-shaped bush in a state that a roller is fit onto the bush are press-fit into the bush holes of a pair of inner plates and then both end parts of the pin having been fit in the bush are press-fit into the pin holes of a pair of outer plates arranged individually on both outer sides of the pair of inner plates. A bushed chain is not provided with such a roller.
In a timing chain of the conventional art, for the purpose of wear resistance improvement of the bush and the pin, chromizing has been applied on the steel base material of the pin.
However, the timing chain had a problem that when used in the engine room of an automobile together with engine oil having severely been degraded, the pin and the bush may easily be worn out so that the lifetime may become short.
Further, in a case that soot generated in the combustion process of the engine gets mixed into the engine oil, since the soot is hard, when the lubricating oil containing the soot enters a space between the pin and the bush serving as the component parts of a timing chain running at a high speed and a high load, wear of the pin and the bush may be accelerated in spite of the formation of the coating film between the pin and the bush.
Thus, surface treatment has been required that improves further the wear resistance of the chain.
For the purpose of resolving a problem that, when a chromium nitride film is formed on a metal surface, heat-cycle spalling easily occurs and hence the adhesion property with the substrate is unsatisfactory, Japanese Patent Application Laid-Open Publication No. H11-29848 discloses a method that: Cr plating is performed on the surface of a metallic material; then the metallic material is heated in a halogenated compound or in a reactive gas containing halogen so that the Cr plating surface is cleaned and activated; and then the metallic material is heated in a nitriding atmosphere so that the Cr plating surface is nitrided and thereby a chromium nitride film is formed.